The jihadists of the so-called "Islamic State" pose a real threat to the West but are "not invincible," and air strikes have exposed their weaknesses, a senior US intelligence official said Wednesday.
Matthew Olsen, director of the National Counter-Terrorism Center, said there is no "credible" evidence that the IS militants fighting in Iraq and Syria are plotting an imminent attack on American soil.
But he acknowledged that the group could eventually try to strike at the United States.
US air strikes in Iraq over the past month have shown the militants are vulnerable on the battlefield, Olsen said, at an event organized by the Brookings Institution think tank in Washington.
"Because of the successes of these strikes, ISIL is losing arms, it's losing equipment and it's losing territory," he said, using the acronym for the group's former name, the Islamic State of Iraq and the Levant.
Iraqi government, Kurdish and US forces taking joint action in recent weeks have, he said, "revealed that ISIL is vulnerable to coordinated and effective military action.
"The strikes have begun to sap ISIL's momentum and created the space for Iraqi and Kurdish forces to take the offensive."
He said the outrage caused by the group's brutal tactics and its setbacks on the battlefield showed IS extremists are "not invincible."
As head of the NCTC, Olsen oversees US intelligence assessments of terror suspects and potential threats to the United States.
Olsen said the Islamic State is merely one of a myriad of extremist organizations around the world, and cautioned against overstating the threat the group represented.
He said it was important to keep the IS threat "in perspective," saying the militants were dangerous but not equivalent to the threat presented by Al-Qaeda before the attacks of September 11, 2001.
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"ISIL is not Al-Qaeda pre-9/11," he said. "At this point, we have no credible information that ISIL is planning to attack the United States."
If "left unchecked," the group would eventually turn their sights to the United States and the West, he added.
Instead of a large-scale attack similar to 9/11, it was more likely an IS member or someone inspired by the group could carry out a smaller scale assault, he said.
Olsen cited the May 2014 attack on a Jewish museum in Brussels as having been carried out by an IS extremist.
Four people died in the shooting and a French national has been extradited to Belgium and charged in the case.
Echoing comments by President Barack Obama, Olsen called for an international coalition to defeat the extremists.
The group triggered global shock and revulsion on Tuesday when it posted another video showing the grisly execution of a an American journalist, Steven Sotloff.
The murder followed the beheading last month of US reporter James Foley.
Obama on Wednesday said Washington would not be intimidated by the murders of Americans but acknowledged it would take time to defeat the IS extremists.
According to Olsen, more than 12,000 foreign fighters had travelled to Syria to battle President Bashar al-Assad's regime, including more than 1,000 Europeans and more than 100 Americans.
"Many of these foreign fighters have joined ISIL's ranks," he said.
These foreigners would one day return to the West "battle-hardened" and radicalized, and the IS could use them to stage terror attacks, he said.